Add error code for tuple struct constructor error

Author: GuillaumeGomez

src/librustc_privacy/diagnostics.rs

@@ -125,4 +125,28 @@ To fix this error, please remove the visibility qualifier when it is not
 required.
 "##,
 
+E0450: r##"
+A tuple constructor was invoked while some of its fields are private. Erroneous
+code example:
+
+```
+mod Bar {
+    pub struct Foo(isize);
+}
+
+let f = Bar::Foo(0); // error: cannot invoke tuple struct constructor with
+                     //        private fields
+```
+
+To solve this issue, please ensure that all tuple's fields are public. Example:
+
+```
+mod Bar {
+    pub struct Foo(pub isize); // we set its field to public
+}
+
+let f = Bar::Foo(0); // ok!
+```
+"##,
+
 }

src/librustc_privacy/lib.rs

@@ -931,9 +931,9 @@ impl<'a, 'tcx, 'v> Visitor<'v> for PrivacyVisitor<'a, 'tcx> {
                         });
 
                     if any_priv {
-                        self.tcx.sess.span_err(expr.span,
-                                               "cannot invoke tuple struct constructor \
-                                                with private fields");
+                        span_err!(self.tcx.sess, expr.span, E0450,
+                                  "cannot invoke tuple struct constructor with private \
+                                   fields");
                     }
                 }
             }